1. Field of the Invention
The present invention relates to an image recording apparatus capable of changing over a recording resolution.
2. Related Background Art
Conventional image recording apparatuses capable of changing over recording resolutions are disclosed in U.S. patent applications Ser. Nos. 563,859 filed Aug. 7, 1990 as a continuation of Ser. No. 350,649 filed May 11, 1989, Ser. No. 392,036 filed Aug. 9, 1989, Ser. No. 407,311 filed Sep. 14, 1989, and Ser. No. 412,986 filed Sep. 26, 1989.
FIG. 8 is a perspective view for explaining an image forming theory in a conventional image printing apparatus. This apparatus includes a photosensitive drum 100 serving as an electrostatic latent image carrier which is rotated at a constant speed in a direction indicated by an arrow.
A semiconductor laser 101 outputs a laser beam 105 which is ON/OFF-modulated on the basis of print information input from a host computer (not shown) or the like. A polygonal mirror 102 is rotated by a scanner motor 103 at a predetermined speed to deflect the laser beam 105 incident through an optical system such as a collimator lens. A latent image is formed on the photosensitive drum 100 through a focusing lens 106 having f-.theta. characteristics.
A beam detector 104 receives the laser beam 105 scanned on the photosensitive drum 100 from the left to the right prior to scanning of an image write area and outputs a beam detection signal to a controller (not shown) to generate a horizontal sync signal BD which defines a write start position in the main scanning direction.
When a printing sequence is started, the laser beam 105 modulated on the basis of image information is incident from the laser 101 to the photosensitive drum 100 serving as a latent image carrier through the rotating polygonal mirror 102. The laser beam 105 is scanned on the photosensitive drum 100 to form a latent image. The latent image is visualized into a toner image by a developing unit (not shown). The toner image is transferred to and fixed on a recording sheet, thereby forming an image.
FIG. 9 is a control block diagram for explaining a control arrangement of the image printing apparatus shown in FIG. 8. A printer controller 200 develops image information received from a host computer or the like into a bit map in an internal memory. Image data 206 and an image clock 207 for latching this image data are output to a printer engine image control circuit 204 in synchronism with the BD signal 212 obtained from a printer engine BD signal detector 213. At the same time, the printer controller 200 performs serial communication with a print engine print control circuit 201 through a serial communication bus 205.
A scanner driver 202 controls a scanner motor 103 shown in FIG. 8 and rotates it at a predetermined speed on the basis of a motor control signal 208 output from the print control circuit 201.
A laser driver 203 controls emission of a laser beam from the semiconductor laser 101 (FIG. 8) on the basis of a laser control signal 209 output from the print control circuit 201.
The image control circuit 204 receives image data 206 output in synchronism with the image clock 207 from the printer controller 200 and outputs a laser drive signal 211 to the laser driver 203.
The image control circuit 204 receives an image control signal 210 from the print control circuit 201 and performs signal processing associated with resolution change-over.
FIG. 10 is a block diagram of an arrangement of the image control circuit 204 (FIG. 9) which is exemplified by a D flip-flop DFF.
FIG. 11 is a timing chart for explaining operations of the respective parts in FIG. 9, and the same reference numerals as in FIG. 9 denote the same parts in FIG. 11.
When the beam detector 104 shown in FIG. 8 receives the laser beam 105, the BD signal detector 213 shown in FIG. 9 outputs the BD signal 212 to the printer controller 200. The image data 206 is output to the image control circuit 204 in synchronism with the image clock 207. In this case, the printer controller 200 serially transmits control information (e.g., a resolution change-over signal) to the print control circuit 201.
In response to this control information, the print control circuit 201 performs change-over processing such as speed change-over of the scanner driver 202. Therefore, an image can be printed at different resolutions in a single printing processor.
In the above apparatus, however, when the resolution is changed from 300 DPI (dots/inch) to 600 DPI, the size of one pixel of a laser beam radiated on the photosensitive drum is not set to be 1/4 the size of the 300 (DPI) resolution, thus forming a larger image. For this reason, when a line obtained by drawing a one-line image is formed, it becomes thick, thus greatly impairing quality of ruler or the like.